The present invention relates to dual fuel engine systems, and more particularly to low gaseous exhaust emission dual fuel engine systems and a method of operating a dual fuel engine system.
Dual fuel engines are well known in the art, as evidenced by U.S. Pat. Nos. 4,603,674 to Tanaka; 4,463,734 to Akeroyd; and 4,527,516 to Foster. Conventional dual fuel engines are built to operate on the Otto cycle and employ compression ignition of an oil charge to burn a gaseous main fuel energy charge. The quantity of diesel oil injected in the pilot charge is normally 4 to 7 percent of the energy required for full load operation of the engine. In the case of the typical dual fuel engine, the diesel fuel injection system is designed so that full load operation can be achieved with diesel fuel oil delivery only, i.e., with no gaseous fuel added to the fuel charge.
Control of engine emissions, particularly NOx emissions, has become a concern in recent years, as evidenced by U.S. Pat. No. 4,306,526 to Schaub et al, of which the present inventor is a co-inventor, and U.S. Pat. No. 4,524,730 to Doell et al.
However, no dual fuel engine system to date has achieved a low emission rate of pollutants, particularly NOx levels, which is acceptable in certain parts of the United States and Europe.